The present invention relates to an thin-film electroluminescent display panel and, more particularly, to a thin-film electroluminescent display panel shielded by a pair of glass substrates and a protective liquid disposed therebetween.
Firstly, a conventionally-shielded electroluminescnet display panel representative of the prior art is illustrated in FIG. 1, wherein the EL display panel comprises a first transparent glass substrate 1, a transparent electrode 2 made of In.sub.2 O.sub.3, SnO.sub.2 etc. formed thereon, a first dielectric layer 3 made of Y.sub.2 O.sub.3, TiO.sub.2, etc., an EL thin film 4 made of ZnS:Mn, and a second dielectric layer 5 made of a similar material of the first dielectric layer 3. A counter electrode 6 is made of Al and is formed on the second dielectric electrode 16 through evaporation techniques. The first dielectric layer 3 is provided by sputtering or electron beam evaporation techniques. The EL thin film 4 is made of a ZnS thin film doped with manganese at a desired amount. An AC electric field is applied to the transparent electrode 2 and the counter electrode 6 to activate the EL thin film 4.
An example of the above structure of the EL display panel was disclosed in, for example, U.S. Pat. No. 3,967,112 "PHOTO-IMAGE MEMORY PANEL AND ACTIVATING METHOD THEREOF" issued on June 29, 1976, assigned to the same assignee.
The first and the second dielectric layers 3, 5 contain inevitably a plurality of pin-holes and micro-cracks during the fabrication steps thereof. The EL thin film 4 is damaged by moisture passing through the pin-holes and the micro-cracks. Thus the EL thin film 4 produces heat owing to the loss of the electroluminescence and is damaged in its electro-optical properties.
FIG. 1 shows a protective structure for the EL display panel, wherein an insulating layer 8 is coated over the EL display panel, and furthermore, a layer of epoxy resin 9 is provided for shielding the insulating layer 8. This protective structure can result in eliminating the above defects. The insulating layer 8 comprises an insulating film made of Si.sub.3 N.sub.4 or Al.sub.2 O.sub.3. The insulating layer 8 is disposed over the EL display panel which is positioned on the substrate 1.
However, there are in the above protective structure critical defaults in that a plurality of pin-holes unfortunately appear in the insulating layer 8 and the epoxy resin 9 owing to fine dusts and alien substances. In accordance with the large sized EL display panel, it is very difficult to provide uniform and nondefective layers including the insulating layer 8 and the epoxy resin layer 9. Even if complete layers of the 8 and the epoxy resin layer 9 are achieved, the EL thin flim 4 is broken down when applied with an electric field because of the expansion of thermally-damaged regions of the EL thin film 4.
The insulating layer 8 and the epoxy resin 9 are subsequently damaged by heat. Moisture passes through the thermally-damaged regions into the EL display panel from the atmosphere to degrade the electro-optical properties of the EL display panel and eventually cause the distruction of the EL display panel. The existence of the moisture mainly lowers the intensity of the El light from the EL display panel.
Accordingly it is very difficult to produce complete and uniform layers suitable for a EL display panel, including the protective layers 8, 9 by the present fablication techniques.